1. Technical Field
This disclosure generally relates to measuring devices and, more particularly, to measuring devices and methods used on computer numerically controlled machines.
2. Description of the Related Art
Computer numerically controlled machines are widely used to cut forms of metal and other materials in operations such as milling, drilling, grinding, broaching, turning, and the like (such operations being termed “cutting operations” generally). In most cases, and in particular when cutting high-precision forms such as gears and engine parts, there is a need to measure the item being cut and/or the tool used in the machine. Such measuring may be conducted either during or after the cutting operation, and may be performed inside or outside of the machine. Measuring generally is conducted to determine whether the size of the form or tool is within a desired degree of tolerance or to determine proper adjustments to the cutting operation. In particular, measurement during a cutting operation is desirable because it allows corrections to be made more rapidly.
Generally, measurements may be taken either within the CNC machine or externally after removal of the item to be measured from the machine. External measurement, while often useful, is disfavored for a number of reasons. External measurements can make it difficult to conduct extended unattended operations, particularly if tool wear is high. In offline gauging, the part is measured outside the machine after machining is complete. Typically, this is carried out in a separate temperature-controlled room. If a machine is kept running during the measuring step, and if the measured part should prove to be out of tolerance, the machine will have kept producing out-of-tolerance parts. It is common, therefore, for machining to cease until the part measurement is complete, thus reducing productivity of the operation. Additionally, if the workpiece is out of tolerance, an adjustment in the tool offset or insert position is required. Depending on the measurement taken in the measuring step, an operator may need to calculate the necessary adjustment and adjust the machine. This can lead to operator error and additional production of out-of-tolerance parts.
In many cases, multiple machines are employed to produce the same part. In this case, operators need to keep track of which parts came from which machine, thus possibly leading to confusion.
The prior art has provided automotive measuring offline gauging systems which can provide automatic feedback. Such systems are typically expensive and complicated.
Conversely, many known in-machine measurement systems are unsatisfactory. There is often a trade-off between robustness of the device and accuracy of measurement. For instance, touch probes, which are suitable for some purposes, in many cases are not sufficiently accurate for high-precision work. This is in part because touch probes depend on the accuracy of the machine. Additionally, touch probes have longer measurement cycles than other measurement devices, and accordingly they are less productive. Other devices, including air gages and linear variable differential transformers (LVDTs), in some cases are not sufficiently robust to withstand the harsh environment within a CNC machine. They are subject to contamination due to swarf, and may lead to incorrect measurements.
One of the inventors of the present application earlier devised a measuring device (certain embodiments of which are disclosed in U.S. Pat. No. 6,901,797) that attempts to address the foregoing. The device disclosed in the '797 patent employs an incompressible fluid, typically a liquid, to achieve an in-machine measurement using known pneumatic measuring techniques. Applicants have identified shortcomings in that device that are addressed by the methods and apparatus described and claimed herein.